Doctrine of Signatures

Doctrine of Signatures, the concept that the key to humanity's use of various plants was indicated by the form of the plant. The red sap of the bloodroot ( Sanguinaria canadensis ), for instance, was believed to cure diseases of the blood, while the fused leaves of boneset ( Eupatorium perfoliatum ) were used to heal broken bones. The concept was employed by the herbalists of the Renaissance, and was accepted until the latter part of the 19th cent.